dry up and modularize layouts

Changelog.md

app/controllers/application_controller.rb

app/controllers/home_controller.rb

app/controllers/posts_controller.rb

app/controllers/registrations_controller.rb

app/controllers/sessions_controller.rb

app/controllers/status_messages_controller.rb

app/controllers/streams_controller.rb

app/controllers/users_controller.rb

app/views/errors/not_public.haml

app/views/layouts/_footer.html.haml

app/views/layouts/_header.html.haml

app/views/layouts/application.html.haml

app/views/layouts/blank.haml

app/views/layouts/centered_with_header_with_footer.html.haml

app/views/layouts/post.haml

app/views/layouts/with_header.html.haml

app/views/layouts/with_header_with_footer.html.haml

app/views/posts/show.html.haml

app/views/status_messages/bookmarklet.html.haml